The invention relates to an optical head adjusting apparatus which adjusts an angle formed between a normal to the face of an optical disk and the optical axis of an optical head in an optical disk apparatus.
An optical head adjusting apparatus of a first prior art is disclosed in the Japanese published unexamined patent application Hei 2-94115.
FIG. 11 is a side view of the first prior art optical head adjusting apparatus. Hereinafter, the configuration and operation of the apparatus will be described with reference to this figure.
A disk motor 49 which rotatably supports an optical disk 33 is fixed to a base frame 50.
An optical head 51 is supported by a guide shaft 52 and a roller 34 which is disposed on a side face of the optical head 51. The roller 34 engages with a groove (not shown) formed on the base frame 50, thereby enabling the optical head 51 to move in a radial direction of the optical disk 33 which is indicated by an arrow 10 (hereinafter, the direction is referred to as "radial direction 10").
An end portion 52a of the guide shaft 52 which is closer to the disk motor 49 is pivotally supported via a leaf spring 35 by a supporting part 36 which is disposed on the base frame 50.
A part of the guide shaft 52 in the vicinity of the other end 52b engages with a groove-like portion (not shown) of a supporting part 5b which is disposed on the base frame 50, so as to be slidable in the direction perpendicular to the base frame 50. The other end 52b of the guide shaft 52 engages with a spiral cam groove 37a of a cylindrical cam gear 37 disposed on the base frame 50.
The first prior art optical head adjusting apparatus operates in the following manner.
The optical head 51 detects the inclination of the optical head 51 with respect to the radial direction 10 of the face of the optical disk 33. Specifically, the optical head detects whether an angle formed by the optical axis 1a of the optical head 51 and the face of the optical disk 33 is 90 deg. or not. If the angle is not 90 deg., the cylindrical cam gear 37 is rotated. The rotating operation of the cylindrical cam gear 37 causes the other end 52b of the guide shaft 52 engaging with the spiral cam groove 37a to vertically move, so that the guide shaft 52 is swung about the supporting part 36 functioning as the fulcrum. When the guide shaft 52 swings, also the optical head 51 supported by the guide shaft 52 is swung. Therefore, the inclination of the optical head 51 can be adjusted so that the optical axis 1a of the optical head 51 forms the angle of 90 deg. with respect to the face of the optical disk 33.
Another optical head adjusting apparatus of a second prior art is disclosed in the Japanese published unexamined patent application Hei 7-320290.
FIG. 12 is a perspective view showing the configuration of main portions of a driving mechanism of the second prior art of the Japanese published unexamined patent application Hei 7-320290.
Referring to FIG. 12, a disk motor 49 which rotatably supports the optical disk 33 is fixed to a base frame 55. An optical head 51 is supported by guide shafts 52 and 53 so as to move in the predetermined radial direction 10 of the optical disk 33.
End portions 52a and 53a of the guide shafts 52 and 53 which are closer to the disk motor 49 are pivotally supported via leaf springs (not shown) by supporting parts 38 and 39 which are disposed on the base frame 55, respectively.
The other ends 52b and 53b of the guide shafts 52 and 53 are supported by a movably supporting mechanism 40 which vertically moves the guide shafts 52 and 53 with respect to the base frame 55.
The optical head adjusting apparatus operates in the following manner.
The optical head 51 detects the inclination of the optical head 51 with respect to the radial direction 10 of the face of the optical disk 33. Specifically, the optical head detects whether an angle formed by the optical axis 1a of the optical head 51 and the face of the optical disk 33 is 90 deg. or not. If the angle is not 90 deg., the movably supporting mechanism 40 vertically moves the guide shafts 52 and 53. The guide shafts 52 and 53 swing about respective supporting parts 38 and 39 which are functioning as fulcrums. When the guide shafts 52 and 53 swing, also the optical head 51 supported by the guide shafts 52 and 53 is swung. Therefore, the angle of the optical head 51 can be adjusted so that the optical axis 1a of the optical head 51 forms the angle of 90 deg. with respect to the face of the optical disk 33.
As the density and capacity of an optical disk are increased, the wavelength of a laser beam of an optical head is made shorter and the NA (numerical aperture) of an objective lens mounted on the optical head is made higher. Therefore, an error of the inclination angle of the optical axis of a laser beam emitted onto the face of the optical disk produces optical aberration in the optical head.
In other words, signal recording and reproducing properties of an optical disk apparatus are largely affected by the error of the inclination angle of the optical axis of the laser beam with respect to the face of the optical disk. In a large-capacity optical disk apparatus, the inclination angle must be adjusted so that an inclination angle error is largely reduced.
Moreover, in order to prevent deteriorations of the signal recording and reproducing properties of the optical disk apparatus, it is essential to conduct the inclination angle adjustment within a movable range of an objective lens used for converging light from the optical head on the optical disk.
Furthermore, "relative gap variation" which is produced as a result of the inclination angle adjustment and which is defined by variation in the distance between the optical head and the optical disk must be reduced to a level as low as possible.
In the afore-mentioned configuration of the first prior art optical head adjusting apparatus disclosed in the Japanese published unexamined patent application Hei 2-94115, the inclination of the optical head 51 with respect to the radial direction 10 of the optical disk 33 can be adjusted. However, the inclination of the optical head 51 with respect to a tangential direction of the optical disk 33 which is perpendicular to the radial direction 10 cannot be adjusted.
In the first and second prior arts of optical head adjusting apparatuses disclosed in the Japanese published unexamined patent applications Hei 2-94115 and Hei 7-320290, the supporting parts 36 of the base frame 50, or the supporting parts 38 and 39 fixed to the base frame 55 are placed at respective positions which are separated from the rotation axis 4a of the disk motor 49. When the guide shafts 52 and 53 swing about the supporting parts 38 and 39 functioning as the fulcrums, for example, the relative gap variation occurs between the optical head 51 and the face of the optical disk 33.
FIG. 13 is a diagram showing the relative gap variation between the optical head 51 and the optical disk 33 in the optical head adjusting apparatus of the second prior art.
Referring to FIG. 13, the positions of the supporting parts 38 and 39 which are disposed on the base frame 55 have a height h and are separated from the rotation axis 4a of the disk motor 49. The horizontal distance between each supporting part 38 or 39 and the rotation axis 4a is indicated by G. It is assumed that the rotation axis 4a of the disk motor 49 is inclined by an angle .theta. with respect to the base frame 55 and oriented in the direction of an arrow 4b. The design value of the relative gap which is the distance between the optical head 51 and the optical disk 33 is indicated by H0.
Under this state, in order to adjust the angle of the optical axis 1a of the optical head 51 with respect to the face of the optical disk 33, the guide shafts 52 and 53 are rotated by the angle .theta. about the upper points 38a and 39a of the supporting parts 38 and 39, respectively. As a result, a relative gap H1 between the optical head 51 and the optical disk 33 after adjustment becomes a value shown in expression 1. Furthermore, a relative gap variation .DELTA.H is indicated by expression 2. EQU H1=H0+G sin .theta.+h(1-cos .theta.) (1) EQU .DELTA.H=G sin .theta.+h(1-cos .theta.) (2)
In the relative gap variation .DELTA.H, the amount of height variation corresponding to the second term (3) of the right side is very small and hence is negligible. However, the amount of height variation corresponding to the first term (4) of the right side cannot be neglected because the movable range of the objective lens is limited to a very short length (for example, .+-.0.8 mm to .+-.1.0 mm). EQU h(1-cos .theta.) (3) EQU G sin .theta.(4)
As described above, in the optical head adjusting apparatuses of the prior art, the positions of the supporting parts 38 and 39 disposed on the base frame 55 are separated from the rotation axis 4a of the disk motor 49. Therefore, such apparatuses have a problem in that, when the inclination of the optical head 51 is adjusted, the relative gap between the optical head 51 and the optical disk 33 is varied and the distance between the optical head 51 and the optical disk 33 is changed, with the result that the optical disk apparatus cannot satisfactorily perform recording and reproducing operations.